Generally speaking, there are two types of wireless systems: code division multiple access (CDMA)-based synchronous systems and time division multiple access (TDMA)-based asynchronous systems. The TDMA-based asynchronous system most widely used in Europe is the global system for mobile communications (GSM) system.
The GSM standard uses a time multiplex method in which the physical transmission channel is divided into time slots of a time duration corresponding to 156.25 consecutive data symbols, wherein eight consecutive slots are assembled to one time frame. In an alternative of this GSM standard the symbol fractions of four consecutive time slots are assembled and added to the first one of the four time slots.